In a TW electroerosion machine, a continuous electrode wire feeds continuously from a supply means to a takeup means and is axially transported along a continuous path of wire travel established between them. Guide means are commonly disposed at the opposite sides of the workpiece to define a straight-line path across a cutting zone and to maintain wire travel in alignment therewith and in machining relationship with the workpiece in the cutting zone. A power supply is provided for passing an electrical machining current, typically in the form of a succession of pulses, between the electrode wire and the workpiece across a machining gap flooded with a machining liquid, e.g. water, to electroerosively remove material from the workpiece. As the process proceeds, the workpiece is displaced transversely relative to the axis of electrode wire along a prescribed feed path under the command, preferably, of a numerical-control (NC) unit, so that a desired cut is formed in the workpiece.
A TW electroerosion machine is generally required to machine a number of cuts or contours in a workpiece or workpieces in a sequence of separate TW operations, each cut or contour most often starting inside the workpiece. This requires the workpiece to be "threaded" with the wire at a start position associated with a cut or contour desired in any such subsequent TW operation. To this end, it has so far been the common belief that the workpiece must have a preformed through-hole at such a start position for accepting the wire and permitting it to extend therethrough in the established continuous path on the machine. It is also desirable that threading be done automatically without the need for manual handling by the operator. Thus, the workpiece has been positioned or repositioned on the machine to align the preformed through-hole with the established straight-line path so that wire can thereafter be threaded automatically by propelling it into, through and out beyond the preformed starting through-hole along that path which must, of course, be unencumbered (see, e.g. U.S. Pat. No. 3,891,819). With the known automatic threading arrangements according to the conventional concept, difficulties arise due to the extreme thinness of the wire (i.e. the wire is normally less than 0.4 mm in diameter or thickness) and its consequent poor self-supporting ability yet coupled with its inherent elastic characteristic. It has been found that very often wire is deflected prior to entry into the small preformed through-hole or otherwise is caught on a wall portion of the through-hole or elsewhere in the threading path necessarily constituted by various discrete guide elements. As a result, it has been the common belief that the chances of having wire to be successfully threaded on a TW machine even highly sophisticated are at most 90% and mostly much less. We have now found that the need for a preformed through-hole can be drastically limited to provide highly efficient automatic threading (AT) and TW operations and that the conventional threading difficulties in the TW electroerosion system can be effectively overcome by eliminating this need, according to the principles of the invention to be described hereinafter.